Magnetic bearing controller

ABSTRACT

It is an object of the present invention to provide magnetic bearing controller which is able to levitate the controlled object stably by using controlled magnetic forces. In the magnetic bearing controller which supplies a controlled current to an electromagnet for levitating a rotating body at a predetermined position, the controller is provided with an eliminator unit for eliminating frequency components of frequency area which is used by the status detector unit. The eliminator unit is inserted between signal amplifier unit and power amplifier unit for generating controlled current.

TECHNICAL FIELD

[0001] The present invention relates to a magnetic bearing controller,and more particularly to a magnetic bearing controller which controls alevitated rotating body actively by controlling magnetic force, which isgenerated by electromagnets by supplying controlled current thereto,whereby the current is controlled by pulse width modulation, and therotating body is controlled in accordance with the status signals, suchas detected displacement sensor signals thereof.

BACKGROUND ART

[0002] Recently, a magnetic bearing device is becoming to be widely usedin the rotary machines in the various fields. The magnetic bearingdevice levitates and supports a rotating body without contact bymagnetic force, which is generated by electromagnet. The advantages ofthe rotary machines equipped with the magnetic bearings are; abrasiondusts free, maintenance free because lubrication oil is not used,high-speed rotatability, and reduction of noises.

[0003] The magnetic bearings are also suitable when these are used inrotary machines, which are disposed in extreme clean atmospheres, suchas clean rooms for semiconductor manufacturing. Because the magneticbearings need no lubrication oil and do not generate abrasion dust, thesemiconductor wafers are prevented from being contaminated. Thereforemagnetic bearings are advantageous when these are used in clean space,vacuumed space, and so on. Especially in the vacuum space, frictioncoefficient of usual mechanical bearings becomes extremely large;therefore the magnetic bearings are suitable because there is no problemfor the magnetic bearings which support the axes without contact.

[0004]FIG. 1 shows a general circuit configuration of a magnetic bearingcontroller, which controls a levitated rotating body actively bycontrolling current supply for the electromagnets. The electromagnetsare disposed around the rotating body nearby for applying magneticforces thereto. When the rotating body is levitated and supported bymagnetic forces generated by electromagnets without contact thereto, thecontrolled object of the system is the rotating body, for example, aposition thereof. The displacement X of the rotating body comparing thecommand position X₀ is detected by status detector unit, namelydisplacement sensor unit 11 in this case. The detected position X iscompared with the command position X₀ in the deviation circuit, and adifference Δx between detected position X and command position X₀ isinputted to compensator unit 12.

[0005] The compensator unit comprises of a control circuit, such as PID(Proportional plus Integral plus Derivative) control circuit, andgenerates an output signal so as to control that the difference Δxbetween detected position X and command position X₀ becomes zero. Anoutput signal of the compensator unit 12 is inputted into signalamplifier unit 13, where the signal is amplified and the amplitude ofthe signal is limited by a limiter circuit(limiter).

[0006] Power amplifier unit 14 generates a controlled current forsupplying to electromagnet 15 corresponding with the output signal ofthe signal amplifier unit 13. The power amplifier unit 14 includes apulse width modulation circuit for supplying controlled current, whichis pulse width modulated thereby into coils of electromagnets 15. Theelectromagnet 15 generates a magnetic attractive force in accordancewith amount of the controlled current. The magnetic attractive force isapplied to the controlled object 16, namely the rotating body in thiscase, and the controlled object is moved so as to be the difference ΔXis decreased to be zero, namely the detected position X moves to thecommand position X₀. By the above mentioned feedback control, thecontrolled object 16 (rotating body in this case) is controlled to bestably positioned at the command position X₀, even if disturbance forceis applied from outwards to the controlled object 15 so as to disturbthe position thereof.

[0007] In the magnetic bearing controller, a displacement sensor isusually employed as the status detector unit 11 for detecting thestatus, which is displacement X comparing to the command position X₀ inthis case. One of typical displacement sensor is an induction typedisplacement sensor, which has a core of magnetic material with coilswound thereof. According to the inductance type displacement sensor, thedisplacement of the controlled object which has a magnetic materialfixed thereon is measured by the detection of variation of theinductance of the coils thereof.

[0008]FIG. 2 shows a detail of signal amplifier unit 13 and a portion ofpower amplifier unit 14, which includes PWM (Pulse Width Modulation)circuits. The output of compensator unit 12 is inputted to the signalamplifier unit 13 which comprises of an amplitude limiter 25, adeviation circuit 29, a signal amplifier 28, another amplitude limiter29 and so on. An output of DC signal generator 26 for setting bias DCcurrent is inputted to the deviation circuit 29 for adding the outputsignal thereto. The power amplifier unit 14 comprises of a PWM circuithaving a comparator circuit 31 and a chopping wave generator 32, and apower amplifier circuit 35 which amplifies the output signal of thecomparator circuit 31 to the actual current to be supplied to the coilsof electromagnets 21.

[0009] The output current of the power amplifier 35 is supplied to thecoils of electromagnets 21 as a controlled current, thereby controlledmagnetic force is generated by electromagnets 21, and applied to thecontrolled object (rotating body) 22 for controlling the positionthereof. The levitated actual position of the controlled object 22 isdetected by the status detector, namely the inductance type displacementsensor 23 in this case. The controlled current which is applied to thecoils of the electromagnets is detected by a current sensor 36, andlower frequency components of the output signal of the current sensorare returned to the deviation circuit 29 by a feedback loop through alow pass filter 39 for stopping higher frequency components which arecorresponding to the frequency components of the PWM signals of thecontrolled current.

[0010] As above mentioned, the signal amplifier unit 13 is comprised ofthe signal amplifier 28 for amplifying an output signal of compensatorunit 12 and the limiter 29 for limiting the amplitude of the amplifiedsignal by the amplifier 28. These circuits are inserted at the front endof the PWM circuit, which comprises of the comparator 31 and thechopping wave generator 32. For driving the coils of electromagnets 21which are inductive load, relatively large gain, for example 10 through100 times amplification is required by the signal amplifier 28 whichamplifies the output signal of the compensator unit 12 for inputting tothe PWM circuit 31 of the power amplifier unit 14.

[0011] However, there is a problem that output signal of the signalamplifier unit 13 is deformed to be a rectangular shaped waveform frominput sine shaped waveform by passing through the deviation circuit 27,the signal amplifier 28, and the amplitude limiter 29. FIGS. 3A through3C show the deformation of waveform and frequency spectrum of thedeformed waveform. FIG. 3A shows an input signal of sine waveform of 1kHz, which is inputted to the signal amplifier unit 13. FIG. 3B shows awaveform of controlled current of 1 kHz corresponding to FIG. 3A whichflows in the coils of electromagnets showing a rectangular shape. Thereason of such deformation of the waveform is estimated that a phasedifference is generated in the deviation circuit 27 between input signaland feedback signal, and that the output signal of the deviation circuitis amplified in the signal amplifier 28 including phase differencestherebetween and saturated therein by the power supply voltage (±15V).Therefore, the input signal of the PWM circuit 31 becomes rectangularshaped waveform from original sine shaped waveform by passing throughthe signal amplifier unit 13.

[0012] Accordingly, the controlled current flowing in the coils ofelectromagnets 21 includes n times harmonic components of fundamentalfrequency of input sine wave, and the frequency spectrum distribution isshown in FIG. 3C. As shown in FIG. 3C, n times harmonic components aregenerated from fundamental frequency of input sine wave of 1 kHz. Thefrequency area where the harmonic components are distributed exceedsmore than 10 kHz as shown by circle in FIG. 3C. Further, frequencyspectrum distributions over 100 kHz are caused by the PWM circuit 31 forgenerating pulse width modulated controlled currents, which aremodulated by for example 90 kHz chopping frequency.

[0013]FIG. 4A shows an input signal of sine waveform of 500 Hz, which isinputted to the signal amplifier unit 13. FIG. 4B shows a waveform ofcontrolled current of 500 Hz corresponding to FIG. 4A which flows in thecoils of electromagnets showing a rectangular shape. FIG. 4C showsfrequency spectrum distribution of the controlled current correspondingto 500 Hz of FIG. 4B. In the same way as 1 kHz input signal form, sinewave input signal is deformed to be rectangular shaped waveform bypassing through the signal amplifier unit 13, and the controlled currentis modulated in accordance with the deformed waveform, which flows inthe coils of electromagnets. The frequency spectrum distributionincludes many n times harmonic components in higher frequency areaexceeding more than 10 kHz. It is a problem that harmonic components aredistributing nearby 10 kHz frequency area, which is shown by a circle inFIG. 4C.

[0014]FIG. 5 shows frequency areas, which are used, in the magneticbearing controller. Area {circle over (1)} is a frequency area of lessthan several kHz which is used by the detected signals of displacementsensor, and original controlled currents in the coils of electromagnets.Namely, area {circle over (1)} is used for controlling the rotating bodywhich is detected by the displacement sensor, and controlled by magneticforces without contact by controlled currents in the coils ofelectromagnets.

[0015] Area {circle over (2)} is a frequency area which is used by theinductance type displacement sensor. The inductance type displacementsensor uses an amplitude modulated signal of, for example, 10 kHzfrequency as a fundamental frequency. In accordance with an amount ofvariation of inductance, the amplitude of the fundamental frequency waveis modulated by the amplitude modulation, and then the position of therotating body is detected by the change of inductance, namely detectingthe amplitude of the modulated signal thereof. Therefore, the frequencyspectrums are distributed nearby 10 kHz by the operation of inductiontype displacement sensor.

[0016] Area {circle over (3)} is a frequency area which is used by thePWM circuit 31 for generating controlled current which is supplied tothe coils of electromagnets. The PWM circuit 31 uses 90 kHz choppingwave as a fundamental frequency, then the PWM circuit 31 generates thefundamental frequency component and harmonics frequency components bythe PWM waveform which are distributed widely over 90 kHz.

[0017]FIG. 6 shows a frequency spectrum distribution of more than 1 kHzon which the frequency spectrum distribution that is shown in FIG. 3C orFIG. 4C is superimposed. The inductance type displacement sensor usesnearby 10 kHz frequency area for detecting the displacement of therotating body by the amplitude modulation of fundamental 10 kHzfrequency. However, the frequency area that is used by the inductancetype displacement sensor is superimposed by n times harmonics componentsof the controlled current in the coils of electromagnets. Thesuperimposed signals of two kinds of frequency areas cause thedisplacement sensor output signal to be deformed or disturbed byharmonics noises of controlled current, and it causes to injure thecontrollability of the magnetic bearing seriously. Therefore, in aserious case, the controller cannot control the rotating body to belevitated.

DISCLOSURE OF INVENTION

[0018] It is therefore an object of the present invention to provide amagnetic bearing controller which is able to levitate the controlledobject stably by using controlled magnetic forces, even if the statusdetector unit employs relatively low fundamental frequency for detectingthe status of the controlled object.

[0019] According to the present invention, there is provided a magneticbearing controller for supplying a controlled current to anelectromagnet for levitating a rotating body at a predeterminedposition, the controller comprising: an electromagnet for generatingmagnetic force by the controlled current; a power amplifier unit forsupplying the controlled current to the electromagnet, the controlledcurrent being pulse width modulated; a signal amplifier unit foramplifying signal before inputting to the power amplifier; a statusdetector unit for detecting a status of the rotating body, the rotatingbody being levitated by magnetic force which is generated by theelectromagnet according to the controlled current; and an eliminatorunit for eliminating frequency components of frequency area which isused by the status detector unit, the eliminator unit being insertedbetween the signal amplifier unit and the power amplifier unit.

[0020] Accordingly, as an eliminator is inserted between the signalamplifier unit and the power amplifier unit for eliminating frequencycomponents of frequency area which is used by the status detector unit,the harmonics components which are generated by the signal amplifierunit, are eliminated by the eliminator. Then the input signal to thepower amplifier unit does not contain the frequency components offrequency area which is used by the status detector unit, and thecontrolled current which is supplied to the electromagnet by the poweramplifier unit also does not contain the frequency components offrequency area which is used by the status detector unit. Therefore, thestatus detector unit is prevented from malfunction, which is caused byharmonics components generated by the signal amplifier unit, and themagnetic bearing controller is able to operate stably and rapidly forcontrolling the controlled object.

[0021] The power amplifier unit is provided with a pulse widthmodulation circuit which comprises of comparator for comparing an inputsignal with chopper wave signal, and the eliminator is connected at thefront end of said comparator. Accordingly, harmonics components of thefrequency area, which is used by the status detector unit, areeliminated at the front end of the pulse width modulation circuit, andthe status detector is prevented from being disturbed by harmonicsnoises which are contained in the controlled current.

[0022] The status detector is an induction type displacement sensor.Accordingly, an induction type displacement sensor is able to be adoptedas the status detector, which is easily available and generally orwidely used in the industry. Furthermore, the fundamental frequency ofthe sensor is able to be lowered for suitably using in such as cannedencapsulated type magnetic bearing.

[0023] The eliminator is a band eliminator filter. Accordingly, theharmonics components of the frequency area which is used by the statusdetector unit, is effectively eliminated easily.

BRIEF DESCRIPTION OF DRAWINGS

[0024]FIG. 1 is a block diagram showing a general circuit configurationof the magnetic bearing controller;

[0025]FIG. 2 is a schematic view showing detail configuration of aportion of the conventional magnetic bearing controller;

[0026]FIG. 3A is a graph of input waveform of 1 kHz sine wave,

[0027]FIG. 3B is a graph of conventional controlled current waveform inthe coil of electromagnet, and FIG. 3C is a graph of conventionalfrequency spectrum distribution of the controlled current;

[0028]FIG. 4A is a graph of input waveform of 500 Hz sine wave, FIG. 4Bis a graph of conventional controlled current waveform in the coil ofelectromagnet, and FIG. 4C is a graph of conventional frequency spectrumdistribution of the controlled current;

[0029]FIG. 5 is a graph showing frequency area {circle over (1)} that isoriginally used by controlled current in the coil of electromagnet,frequency area {circle over (2)} that is used by the status detector(displacement sensor), and frequency area {circle over (3)} that is usedby PWM waveform of the controlled current;

[0030]FIG. 6 is a graph showing the frequency area, where the frequencyarea {circle over (1)} and the frequency area {circle over (2)} aresuperimposed with each other according to the conventional controller;

[0031]FIG. 7 is a schematic view showing detail configuration of aportion of an embodiment of the magnetic bearing controller according tothe present invention;

[0032]FIG. 8 is a schematic view showing detail configuration of aportion of signal amplifier unit and power amplifier unit of themagnetic bearing controller shown in FIG. 7;

[0033]FIG. 9A is a circuit diagram of the PWM circuit and FIG. 9B is agraph showing the operation of the circuit shown in FIG. 9A;

[0034]FIG. 10 is a graph showing frequency characteristics of theeliminator shown in FIG. 7;

[0035]FIG. 11A is a graph of input waveform of 1 kHz sine wave, FIG. 11Bis a graph of controlled current waveform in the coil of electromagnet,and FIG. 11C is a graph of frequency spectrum distribution of thecontrolled current, according to the present invention respectively; and

[0036]FIG. 12A is a graph of input waveform of 500 Hz sine wave,

[0037]FIG. 12B is a graph of controlled current waveform in the coil ofelectromagnet, and FIG. 12C is a graph of frequency spectrumdistribution of the controlled current, according to the presentinvention respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

[0038]FIG. 7 shows an embodiment of circuit configuration of the signalamplifier unit and the power amplifier unit according to the presentinvention. As shown in FIG. 7, the magnetic bearing is comprised ofelectromagnet 21 for producing magnetic force and rotating body 22 whichis levitated and supported by the magnetic force generated by theelectromagnet 21, thereby the levitated position of the rotating body 22is controlled actively by controlled current which is supplied by thepower amplifier unit 14. FIG. 7 shows only a portion of controllercircuit configuration, namely an electromagnet 21 and correspondingcircuit portion thereto. An inductance type displacement sensor 23 isbeing disposed nearby the rotating body 22 for detecting the position Xthereof. The output signal of the displacement sensor 23 is inputted tothe compensator unit 12 (not shown in FIG. 7) for generating controlsignal so as to keep the levitated position of the rotating body 22 atcommand position, as described before with referring to FIG. 1.

[0039] The output signal of the compensator unit is inputted to theamplitude limiter circuit 25, and the amplitude of the signal is limitedin a range as shown in FIG. 7. The output signal of the limiter 25 isinputted to the deviation circuit 27, where DC signal of DC signalgenerator 26 for setting bias current is added thereto for giving biasvoltage. The output signal of the deviation circuit 27 is inputted tothe signal amplifier 28 where the signal is amplified by the gain of10-100 times amplification.

[0040] Actual circuits are shown in FIG. 8, DC signal generator 26,deviation circuit 27 and signal amplifier 28 are integrally configuredby using operational amplifiers, and have functions of adding,amplifying and amplitude limiting. The amplitude limiter circuit(limiter) 25 comprises of operational amplifier, zener diode, andresistance circuit for dividing the applied voltage. The limiter 25produces a function that if amplitude of input signal is larger than apredetermined range, then the amplitude is limited to the predeterminedvalue of the range. DC signal generator 26 for setting bias currentcomprises of DC voltage signal generator having variable resistor suchas volume for adjusting DC voltage or DC current. The deviation circuit27 comprises of (−) side terminal portion of the operational amplifier28 a. The output voltage terminal of DC signal generator 26, outputsignal line of the limiter 25 and output signal line of current feedbackunit 39 for producing feedback signal of controlled current areconnected to the input terminal 27 of the operational amplifier 28 a.The signal amplifier 28 comprises of a combination of operationalamplifier 28 a and resistors, which are connected thereto. Also, thelimiter 29 comprises of a combination of zener diode and operationalamplifier 28 a.

[0041] The comparator 31 generates PWM signal by comparing input signalwith output signal of chopper wave generator 32. FIG. 9A shows a circuitconfiguration of the pulse width modulator 31, in which comparator 20compares input signal with reference signal of chopper waveform, thusgenerates PWM signal waveform. As shown in FIG. 9B, the intervalsbetween cross points of input signal and reference signal arediscontinuously outputted with DC amplitude to form the PWM waveform.Therefore, amplitude of analog input signal to the power amplifier unit14 is modulated by chopper wave reference signal to generate pulse widthmodulated output signal. Thereafter, the signal is amplified by anamplifier, which is comprised of transistors Q₁ and Q₂ to form outputPWM signal.

[0042] Namely, as shown in FIG. 9B, the pulse width modulation iscarried out by comparing an amplitude of input signal Va with referencechopper wave signal Vb in the comparator 20. When reference chopper wavesignal Vb is higher than the input signal Va, then +Vcc is outputted.When reference chopper wave signal Vb is lower than the input signal Va,then −Vcc is outputted. A frequency of 90 kHz is adopted as thefundamental frequency of the chopper wave reference signal as mentionedbefore.

[0043] An eliminator 30 for eliminating frequency components of thefrequency area, which is used by status detector unit, is insertedbetween output terminal of limiter 29 and input of comparator(PWMmodulator) 31. The eliminator 30 comprises of band eliminate filter(BEF), low pass filter (LPF), or so on. The output signal of thecomparator 31 is inputted to power amplifier 35 via an optical isolator.In the power amplifier 35, controlled current is generated and suppliedto coils of electromagnets for controlling the status of the controlledobject.

[0044] Current detector 36 detects the controlled current. The detectedcontrolled current signal is feedbacked by current feedback unit 39 tothe deviation circuit 27, and subtracting action is carried out therein.The current feedback unit 39 detects low frequency components of thecontrolled current, which flows in the coils of electromagnets, andfeedback the detected frequency components to the input of signalamplifier 28. The current feedback unit 39 comprises of a currentdetecting and amplifying unit 28 a, a low pass filter (LPF) 28 b, anoffset adjusting unit 28 c, a gain adjusting unit 28 d and so on. Here,low pass filter (LPF) 28 b eliminates frequency components of PWMswitching frequencies of the controlled current.

[0045]FIG. 10 shows a frequency characteristics of gain and phase of theband eliminator filter 30, which is inserted at the front end of the PWMmodulator 31. In this example, an attenuation of −31 dB is attained at9.55 kHz. This frequency bandwidth of the attenuation coincides with thefrequency bandwidth, which is used by the status detector unit, namelythe inductance type displacement sensor in this embodiment.

[0046] Accordingly, the frequency area of fundamental frequency andnearby used by the inductance type displacement sensor is eliminated bythe band eliminator filter 30. As mentioned before, the output waveformof the signal amplifier 28 and the limiter 29 is deformed from the inputsignal of sine waveform to be rectangular shaped waveform includinglarge amount of n times harmonics components. Then, in the frequencyspectrum distribution, the harmonics components of the controlledcurrent extend to the frequency area which is used by the inductancetype displacement sensor as shown in FIG. 3C or FIG. 4C. By the bandeliminator filter 30, the harmonics components of the output signal ofthe signal amplifier unit 13 are eliminated at nearby 10 kHz, and thenthe input signal to the power amplifier unit 14 does not contain thefrequency components of such frequency area. The signal now does notcontain harmonics components, which coincide with the frequency areathat is used by the status detector unit. Then the signal is pulse widthmodulated by comparator 31 and supplied to the coils of electromagnets21 via power amplifier 35 as controlled current for generatingcontrolled magnetic force applied to the rotating body. Thus, theinductance type displacement sensor as a status detector unit does notdetect the frequency components of the frequency area of nearbyfundamental frequency thereof, and is able to detect the displacement ofthe rotating body 22 without any disturbance by harmonics components ofcontrolled current.

[0047] Accordingly, the problem is solved that harmonics components ofthe controlled current fall on the frequency area that is used by thedisplacement sensor as shown in FIG. 6. Both frequency areas of area{circle over (1)} and area {circle over (2)} are clearly separated asshown in FIG. 5. Then the inductance type displacement sensor isprevented from being disturbed by the harmonics components, andmalfunctional operations are prevented. It gives the controller tocontrol the rotating body stably to be levitated at the command positioneven though outer disturbance is applied.

[0048]FIGS. 11A through 11C show experimental results of the magneticbearing controller which inserts the eliminator in front of the PWMmodulator therein. FIG. 11A shows an input signal waveform of 1 kHz sinewave of the signal amplifier unit. FIG. 11B shows a waveform ofcontrolled current, which flows in coils of electromagnets correspondingto the input signals of FIG. 11A. FIG. 11C shows frequency spectrumdistribution of the controlled current corresponding to the controlledcurrent of FIG. 11B. As shown in FIG. 11B, the edge of the rectangularshaped waveform becomes rounder shaped with loosing square portionthereof, comparing to FIG. 5B, and closing to the waveform of input sinewave form. Also as shown in FIG. 11C, frequency components of nearby 10kHz frequency area are respectively reduced in the frequency spectrumdistribution, and the frequency area is used by the inductance typedisplacement sensor.

[0049]FIGS. 12A through 12C show the same experimental result when inputsignal is a sine wave of 500 Hz. As shown in FIG. 12B, the edge portionof rectangular waveform becomes rounder than the edge portion of FIG.11B. As shown in FIG. 12C, harmonics components of controlled currentare remarkably reduced at more than 1 kHz frequency area in frequencyspectrum distribution, and almost zero at the frequency area nearby at10 kHz which is used by the inductance type displacement sensor.

[0050] When the magnetic force is applied to the rotating body throughthin stainless steel sheet, for example, in can encapsulated magneticbearing, the fundamental frequency of inductance type displacementsensor should be lower to avoid eddy current loss by the stainless steelsheet. Therefore, frequency area {circle over (1)} and frequency area{circle over (2)} become closer as shown in FIG. 5, and it causes aproblem that harmonics noises of frequency area {circle over (1)} fallon frequency area {circle over (2)} to disturb stable operation ofmagnetic bearing. In some serious cases, controlled current amplified bypower amplifier unit contains harmonic noises in the frequency area{circle over (2)} so many that the magnetic bearing cannot work tolevitate the rotating body. According to the present invention, harmoniccomponents of controlled current are eliminated in frequency area{circle over (2)} which is used by the inductance type displacementsensor. Therefore, it enables to use relative low frequency area {circleover (2)} of about 10 kHz as fundamental frequency of inductance typedisplacement sensor without causing any problem.

[0051] In the above mentioned embodiment, single band eliminating filterhaving about −30 dB attenuation is inserted, however it may be possibleto insert the filters at plural steps. It will provide more perfectelimination of n times harmonic components of controlled current byincreasing attenuation and bandwidth thereof. It is also possible toadopt low pass filter (LPF) instead of band eliminate filter (BEF), andthen the same results are expectable.

[0052] According to the present invention, an eliminator is insertedbetween the signal amplifier unit and the power amplifier unit havingPWM circuit for eliminating frequency components of frequency area,which is used by the status detector unit. The harmonic components ofthe controlled current are prevented from being fallen on the frequencyarea, which is used by the status detector unit. Therefore, it enablesstable operation of magnetic bearing controller in which rotating bodyis levitated by controlled magnetic force at exact command position evenif outer disturbing forces are applied thereto.

Industrial Applicability

[0053] The present invention is useful for magnetic bearings, which areinstalled in the rotary machines for supporting the rotating bodywithout contact. The rotary machines having magnetic bearings aresuitable for use in many industrial applications.

1. A magnetic bearing controller for supplying a controlled current toan electromagnet for levitating a rotating body at a predeterminedposition, said controller comprising: an electromagnet for generatingmagnetic force by said controlled current; a power amplifier unit forsupplying said controlled current to said electromagnet, said controlledcurrent being pulse width modulated; a signal amplifier unit foramplifying signal before inputting to said power amplifier; a statusdetector unit for detecting a status of said rotating body, saidrotating body being levitated by magnetic force which is generated bysaid electromagnet according to said controlled current; and aneliminator unit for eliminating frequency components of frequency areawhich is used by said status detector unit, said eliminator unit beinginserted between said signal amplifier unit and said power amplifierunit.
 2. A magnetic bearing controller according to claim 1, whereinsaid power amplifier unit is provided with a pulse width modulationcircuit which comprises of comparator for comparing an input signal withchopper wave signal, and said eliminator is connected at the front endof said comparator.
 3. A magnetic bearing controller according to claim1, wherein said status detector is an inductance type displacementsensor.
 4. A magnetic bearing controller according to claim 1, whereinsaid eliminator is a band eliminator filter.